The present invention relates generally to exposure apparatuses, and more particularly to a catoptric reduction projection optical system, an exposure apparatus, and a device fabricating method, the catoptric reduction projection optical system using ultraviolet light such as extreme ultraviolet (“EUV”) light to project and expose an object such as a single crystal substrate for a semiconductor wafer (plate or ball), and a glass plate (wafer) for a liquid crystal display (LCD).
Along with the recent demand on smaller and lower profile electronic devices, fine semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, a design rule for a mask pattern requires that an image with a size of a line and space (L&S) of less than 0.1 μm be extensively formed and predictably, it will move to a formation of circuit patterns of less than 80 nm in the near future. L&S denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus as a typical exposure apparatus for fabricating semiconductor devices includes a projection optical system that projects and exposes a pattern of a mask or a reticle (which are used interchangeably in the present application) onto a wafer. Resolution R of a projection exposure apparatus (a minimum size which enables a precise transfer of an image) can be given by using a light-source wavelength λ and the numerical aperture (NA) of the projection optical system as in the following equation:
                    R        =                              k            1                    ×                      λ            NA                                              (        1        )            
Therefore, the shorter the wavelength becomes, and the higher the NA increases, the better the resolution becomes. In recent years, it is required that the resolution be a smaller value and thus, there is a limit in meeting this requirement just by increasing NA. It is thus expected to improve the resolution by shortening the wavelength. Currently, an exposure light source is in transition from a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm) to an F2 excimer laser (with a wavelength of 157 nm), and practical use of extreme ultraviolet (EUV) light is being promoted as a light source.
As a shorter wavelength of light would limit usable glass materials for transmitting the light, use of reflecting elements, i.e., mirrors for a projection optical system are advantageous instead of using many refracting elements, i.e., lenses. No glass materials are usable for EUV light as exposure light, and thus a projection optical system could not include any lenses. It has thus been proposed to form a projection optical system only with mirrors.
In a catoptric reduction projection optical system, a multilayer film is formed on mirrors so that reflected light may be intensified for a higher reflectance on the mirrors, but it is desirable to use as few mirrors as possible so as to increase its reflectance for the whole optical system. In order to prevent a mechanical interference between the mask and the wafer, the number of mirrors is desirably an even number, the mirrors making up the projection optical system such that the mask and the wafer are located at opposite sides with a pupil in between. A smaller critical dimension (or resolution) for the EUV exposure apparatus than a conventional one requires a large NA (e.g., NA of 0.2 for a wavelength of 13.5 nm), while it is hard for the conventional 3 to 4 mirrors to decrease the wave aberration. For the increased degree of freedom in correcting the wave aberration, the increased number of mirrors are needed as well as making the mirrors aspheric. As a result, the projection optical system comes to require so many as six mirrors (while the instant application calls such an optical system a six-mirror system).
However, the conventional catoptric projection optical system of a six-mirror system has not yet realized a well-balanced reconcilement between the high NA and high imaging performance for the EUV light, and disadvantageously it could not provide high quality devices with good exposure performance such as resolution.
For example, U.S. Pat. No. 5,686,728 discloses an embodiment that uses six aspheric mirrors. This reference points out a defect due to a short wavelength of the EUV projection system, and describes an optical system effective for use with a wavelength ranging from 100 nm to 300 nm such as 126 nm, 146 nm, 157 nm, 172 nm, 193 nm, etc., all of which are longer in wavelength than the EUV light. However, this optical system uses a wavelength one digit larger than the EUV light, and thus the wave aberration becomes one digit larger if applying to EUV. This reference as it is would provide such poor imaging performance that it cannot be applied for the EUV light. U.S. Pat. No. 5,815,310 and Japanese Laid-Open Patent Application No. 2000-235144, etc. disclose a six mirror system having the high NA (e.g., NA of 0.15 to 0.25) for the EUV light, but one of their mirrors in the six-mirror system would cause an interfere (or vignette) with light, possibly deteriorating the imaging performance.
Further, the conventional projection optical system using mirrors is so non-telecentric on an object side that a principal ray of light entering and exiting from the reticle or mask (collectively referred to as “reticle” hereinafter) is inclined greatly to a normal line of the object plane. As a result, relative arrangement between the reticle and the wafer (image plane) offsets in the optical axis direction during the scan exposure operation would change the imaging reduction on the wafer, deteriorating the imaging performance.